There is an increased interest in using miniature image devices carried on the head of an observer, head-mounted displays (HMD), to present information to the observer in a dynamic mode. The image device cannot be viewed directly, when placed before the observer's eye, due to the closeness of the device to the observer. Optical elements are used to create a virtual image of the image device at some distance so as to be comfortably viewed by the observer. One of the challenges of building such a device is to make the optical elements compact while maintaining optical performance that will support the resolution of the imaging device. While some applications having a modest field of view that can be supported by magnifier viewer wider field of view devices using small miniature image devices, they require compound optical systems. Compound optical systems that preserve a direct view of the image source are commonly known as pupil forming systems.
The present disclosure relates to compound optical pupil forming systems. Such devices are more fully described by Shenker in U.S. Pat. No. 3,432,219 ('219), hereby incorporated herein by reference. The devices in the '219 patent consist of two main parts, a relay lens and an eyepiece lens combined to form an erecting eyepiece.
As illustrated herein by FIG. 1, Chen et al discloses in U.S. Pat. No. 5,822,127 ('127), hereby incorporated by reference, the form configured for a helmet display, 100. The light path begins at the image plane (102) and is relayed by (104, 104a, 104b) forming a real image near a fold mirror (106). The light is then collimated by (108, 108a) and is observed by the eye (122) at the conjugate pupil (120). When the image plane is a reflective display, additional optics are required for illuminating the display.
A typical illumination arrangement is illustrated, by Weissman et al., in U.S. Pat. No. 5,984,477 ('477) in FIG. 2 (FIG. 1 in the '477 patent). The '477 patent is hereby incorporated by reference.
Referring to FIG. 2 from Weissman et al., a system 200 includes having an image formed on the spatial light modulator (SLM) 202, is a ferroelectric liquid crystal (FLC) device, and is projected onto the rear projection screen 204 by the lens assembly 206. The SLM 202 is illuminated by light source 208, which is focused onto the SLM 202 by lenses 210, 212 and 214 via polarizing cube beam splitter (PBS) 216. The light source 208 may comprise a lamp or an optic fiber cable relaying light from a lamp to lens 210. The lenses 210, 212 and 214 results in a low numerical aperture, collimated beam which is directed normally onto the SLM 202.
The polarizing cube beam splitter 216 reflects light polarized in one direction and transmits light polarized in the orthogonal direction. Light impinging on a pixel in the OFF state is reflected back with the same polarization and re-enters the illumination system via lenses 214, 212 and 210. Light impinging on a pixel in the ON state is reflected back with its plane of polarization rotated 90 degrees, and is, therefore, transmitted by the polarizing beam splitter toward the rear projection screen 204. Ferroelectric liquid crystal devices and their operation are known in the art.
Polarizers 218 and 220 serve to reduce unwanted light reaching the rear projection screen 204 and consequently increase the contrast of the image. The first polarizer reduces the amount of light entering the cube 216; however, substantially all of the polarized light entering the cube is reflected by the polarized reflective surface inside cube 216. Although the reflective surface is fully reflective (and not half reflective) for polarized light, the term “beam splitter” is used since splits non-polarized beams into polarized beams.
The image on the rear projection screen 204 is viewed through the cube eyepiece shown generally at 222 by an observer placing his or her eye at viewpoint 224. The basic eyepiece is formed by beam splitter 226 and the spherical mirror 228 which serves to create a magnified virtual image of the rear projection screen 204 at a relatively large distance from the observer. Lenses 230 and 232 provide color correction while lens 234 helps to achieve uniform brightness at the normal viewing position.
FIG. 3 illustrates an embodiment described in U.S. Pat. No. 5,596,451 ('451) by Handschy et al. that illustrates how a cube beam splitter may utilize all 4 sides in an optical design.
Illustrated as assembly 300 of FIG. 3 herein, assembly 300 includes illumination arrangement 302, spatial light modulator 304, and an optics arrangement 306. The optics arrangement 306 includes a first member, specifically a mirror 308 having a curved light reflecting surface 310 which are configured to, in cooperation with other members of optics arrangement 306, direct light into a predetermined area 312. Optics arrangement 306 also includes a second member, which in this embodiment is a polarizer-analyzer beam splitting cube 314, hereinafter referred to as polarizing beam splitting cube 314, having a plurality of external surfaces or faces. The illumination arrangement 302 is positioned in proximity to and in optical communication with a first external face 316 of cube 314. If illumination arrangement 302 produces light which is not polarized, an auxiliary polarizer 318 is positioned between illumination arrangement 302 and face 318 of cube 314. Illumination arrangement 302 can be readily removably attached adjacent to face 318 of cube 312 to allow for replacement or repair of this component, as indicated generally at 320. Also, spatial light modulator 304 is positioned in proximity to and in optical communication with a second external face 322 of cube 314 and mirror 308 is positioned in proximity to a third face 324 of cube 314 and a quarter wave plate 326 is positioned between mirror 308 and face 324 of cube 48. In this preferred embodiment of the present invention, mirror 308 and/or spatial light modulator 304 are readily adjustably attached adjacent to face 322 and/or face 324, respectively, as indicated generally at 328. This arrangement allows the distance between mirror 308 and face 324 of cube 314 and/or the distance between spatial light modulator 304 and face 322 of cube 314 to be adjusted within a predetermined range of distances thereby providing means for focusing the image generated by the assembly.
Polarizing beam splitting cube 314 includes a polarizing beam splitting film or layer 330 positioned within cube 314 such that one side of film 330 faces external faces 316 and 322 of cube 314, and the other side of film 300 faces external face 324 and a fourth external face 332 of cube 314. As indicated by lines 334 and 336, which represent light provided by illumination arrangement 302, light produced by illumination arrangement 302 is linearly polarized by auxiliary polarizer 318 such that S-polarized light is directed into film 330 within cube 314. It is to be understood that fines 334 and 336 and all other lines subsequently used to trace light through the assemblies are illustrative only and are not intended to represent a ray trace as is commonly performed in the course of an optical design. It is also to be understood that the term S-polarized light is used in the common manner wherein it specifies that the electric vector of the light incident on a reflective surface is perpendicular to the plane of incidence, in this case the plane of the drawing.
Since film 330 is a polarizing beam splitting film, the majority of the S-polarized light 332 is directed into spatial light modulator 304. Spatial light modulator 304 is a reflective spatial light modulator having a reflective surface and a light modulating medium, in this case a ferroelectric liquid crystal layer, which is switchable between different states. The reflective surface and the modulating medium cooperate to act on light in ways that form an overall pattern of reflected, modulated light, which constitutes a modulation encoding of a picture which may be viewed. For this embodiment, the S-polarized light which is directed into spatial light modulator 304 is modulated by the ferroelectric liquid crystal material such that the overall pattern of reflected, modulated light is a pattern of light of S-polarized light and P-polarized light which is orthogonally polarized to the S-polarized light. At any point in this pattern, the polarization depends on the state of the corresponding pixilated portions of the ferroelectric liquid crystal material through which the S-polarized light from illumination arrangement 302 has passed. Spatial light modulator 304 directs this modulated light back into cube 314 where the light is analyzed by polarizing beam splitting film 330, as will be described immediately below.
The purpose of analyzing the pattern is to decode the polarization modulated pattern and transform it into a brightness modulated pattern which can be viewed and recognized as a display image. As indicated by line 336, the S-polarized light from illumination arrangement 302 which spatial light modulator does not change, and therefore remains S-polarized light, is directed back toward illumination arrangement 302. As indicated by line 334, the S-polarized light from illumination arrangement 302 which spatial light modulator changes to P-polarized light passes through film 330 and is directed toward mirror 308 through quarter wave plate 326. Mirror 308 reflects light 334 back through quarter wave plate 326 which, since light 334 has passed through quarter wave plate 326 twice, changes light 334 back to S-polarized light. And finally, polarizing beam splitting film 330 directs this S-polarized light out of cube 314 through external face 332 into area 312 which extends outwardly from face 332.
The components of the above described arrangement are mutually disposed and the curvature of mirror 308, which in this case is a magnifying mirror, is established so as to produce a viewable magnified image of the pattern of modulated light created at and by spatial light modulator 304. This image is viewable when a viewer places an eye within viewing area 46 which extends outward from the fourth face 66 of cube 48 and when the eye is directed generally toward face 322 of the cube. This viewable image is made luminous by light from illumination arrangement 302 as modulated by the polarization control affected by spatial light modulator 304 in cooperation with polarizing beam splitter film 330 and auxiliary polarizer 318, if included.
While the prior art completes the task of forming a virtual collimated image for a helmet display optical system the components are spread out over a physical large area. Additionally, components such as the polarizing cube beam splitter 216 in the Weissman '477 patent are not used to full advantage as only three of the four available sides are utilized in the optical design. The majority of the lens elements are also used one time. There is a need for a compact optical with its total length, width and height significantly shorter than its optical path.